The requirement's usually tied to holding the title, like they want that to have been your full-time thing. It's been my experience that people see what you're doing in a very different light depending on your title (one title, one company one day, everyone second-guesses or ignores everything you say; different title, different company, a few days later, suddenly everyone's deferring to you in meetings and sincerely asking your opinion about everything to such a degree that it's unnerving) including in hindsight—if your title was "lead" you don't have to spend a ton of time explaining exactly how, as a non-"lead", you were leading to convince people you were, and you don't have to avoid giving the impression you were being "bossy" or overstepping by doing it.
[EDIT] my suspicion here is that yeah, it's basically "luck into it". I mean that's how everything else I've done career- and pay-progression-wise has worked (luck into someone tasking me with something they definitely wouldn't hire me to do, but after I've done it for a few months someone else would) so maybe this'll work out the same way.
